Discrete multitone modulation (DMT)—also called multicarrier modulation—is a modulation method which is suitable in particular for the transmission of data via channels effecting linear distortion. Compared with a so-called single-carrier method—for example amplitude modulation—which has only one carrier frequency, a multiplicity of carrier frequencies are used in discrete multitone modulation. Each individual carrier frequency is modulated in amplitude and phase according to the quadrature amplitude modulation (QAM). A multiplicity of QAM-modulated signals are thus obtained. In this case, a specific number of bits can be transmitted per carrier frequency. Discrete multitone modulation is used for example for digital audio broadcasting DAB under the designation OFDM (Orthogonal Frequency Division Multiplex) and for the transmission of data via telephone lines under the designation ADSL (Asymmetric Digital Subscriber Line).
In ADSL, the physical transmission channel is a two-wire line (copper double core) of the telephone network. However, such a transmission channel has a long transient recovery time. Signals generated by discrete multitone modulation typically contain very short pulses having a high amplitude, which effect impulse responses that decay slowly in this transmission channel. If an impulse response has still not completely decayed when a new pulse arrives at the receiver, then interference occurs in the receiver. For compensation of such interference, DMT receivers 0contain time domain equalizers, for example, which are intended to shorten the impulse response of the transmission channel and avoid interference on account of superposition of an impulse response of a pulse that has not yet decayed and an impulse response of a subsequent pulse.
The time domain equalizer (TDEQ) may be embodied for example as a digital transversal filter whose coefficients are adjustable. The design of such time domain equalizers is described in Al-Dhahir, N., Cioffi, J. M., “Optimum Finite-Length Equalization for Multicarrier Transceivers”, IEEE Trans.on Comm., Vol. 44, No. 1, January 1996.
The document U.S. Pat. No. 5,521,908 describes a method for determining a set of time domain parameters of an SIRF filter. To that end, the original channel and echo impulse responses are determined to an approximation and recorded on SIRF coefficients based on the combined channel and echo impulse responses that have been calculated to an approximation. An SSNR ratio is calculated for the SIRF coefficients, the individual steps being repeated for determining the coefficients with the best SSNR ratio.
The document EP 0 768 778 A1 comprises a method and a corresponding apparatus for the transmission of impulse responses. A set of parameters is calculated for an equalizer, which equalizes an impulse response in such a way that the equalized impulse response corresponds, to an approximation, to a desired impulse response of a predetermined length. To that end, an error function is minimized by means of eigenvalue and eigenvector calculation of a channel-dependent matrix. The channel-dependent matrix comprises a signal, a disturbed signal, a desired impulse response length and a desired impulse response delay. The error function has a first component, which represents the difference between an equalized impulse response and the desired impulse response, and a second component, which represents the energy transmitted in unused frequency bands. The eigenvector associated with the minimum eigenvalue of the channel-dependent matrix represents the set of equalizer parameters.
What is disadvantageous with such time domain equalizers, however, is the high number of coefficients of the digital transversal filter used as time domain equalizer, and the complex adaptation of the digital transversal filter. Given a filter length of 20 to 40 coefficients, approximately 50 to 100 million multiplications have to be carried out per second. Accordingly, a digital filter for time domain equalization requires a very high computing power. In addition, each coefficient has to be adjusted for the adaptation of the digital transversal filter. This requires a long adaptation time which has to be provided at the beginning of an ADSL transmission.